Dual reflector antenna capable of steering radiated beams

ABSTRACT

In a dual reflector antenna comprising a main reflector and a subreflector, the reflector curves of the main reflector and the subreflector are so constructed that when the subreflector is inclined at two angles relative to the main reflector, the radiated beams are in phase in two different directions slightly angularly spaced apart from each other. In steering the radiated beams, the angle of inclination of the subreflector is continuously varied while simultaneously the distance between the main reflector and the subreflector is varied.

o Umted States Patent 1 [111 3,745,582

Karikomi et al. July 10, 1973 [54] DUAL REFLECTOR ANTENNA CAPABLE 2,419,556 4/1947 Feldman 343/837 X 0 ING RADIATED BEAMS 3,430,244 2/1969 Bartlett et al.. 343/837 3,562,753 2/1969 Tanaka et al. 343/837 Inventors; Masahlro Karikoml, Yoshw 3,394,378 7/1968 Williams et al.... 343/836 x Kataoka, bOth'Of Tokyo, Japan 3,438,041 4/1969 Holtum, Jr 343/836 [73] Assignee: Nippon Telegraph and Telephone i Public Corporation, Tokyo, Japan Primary Examiner-Rudolph V.' Rolinec Assistant Examiner--Saxfield Chatmon, Jr. [22] Flled' Sept 1971 Attorney-Milton J. Wayne et al. [21] Appl. No.: 180,639

[30] Foreign Application Priority Data [57] ABSTRACT Sept. 28, 1970 Japan 45/84107 In a duaI reflector antenna comprising a main reflector and a subreflector, the reflector curves of the main re- [52] U.S. Cl 343/758, 343/832, 343/837, flector and the subreflector are so constructed that 343/839 when the subreflector is inclined at two angles relative [51] Int. Cl. H01q'3/00 to the main reflector the radiated beams e in phase [58] Fleld of Search 343/758, 836, 837, in two different directions slightly angularly Spaced 343/839' 761 apart from each other. in steering the radiatedbeams,

the angle of inclination of the subreflector is continu- [56] References Cited ously varied while simultaneously the distance between UNITED STATES PATENTS I the main reflector and the subreflector is' varied. 3,370,295 2/1968 Kelsey 343/837 X I 1 v 6 Claims, 11, Drawing Figures 8 Sn r' A PATENTEU'JUL 1 0 ma SHEEI 3 0F 5 ANGLE 0 (deg) FIG. 6

"YEMEN-10.973 3.745.582

' m s or s ERROR N90 DETECTOR ERROR SIGNAL T ERROR SIGNAL GENERATOR GENERATOR 2 J1 ANALOG T COMPUTER 93 v T CONTROL CONTROL CONTROL CIRCUIT CIRCUIT CIRCUIT DUAL REFLECTOR ANTENNA CAPABLE OF STEERING RADIATED BEAMS BACKGROUND OF THE INVENTION The present invention relates to generally a dual reflector antenna including a main reflector and a subre'- flector and more particularly a dual reflector antenna in which a main reflector is held stationary while a subreflector is inclined to track a satellite especially a geostational satellite.

Recently the so-called satellite communication system has been widely used in which a communication satellite or satellites are used as repeaters for longrange communication systems between earth stations spaced apart from each other by a very long distance. In order to track a communication satellite, a directive antenna in an earth station is generally supported on a mount so that the antenna may be tilted and rotated in response to the movement of the satellite. A reflector of the antenna is very large in diameter and very heavy in weight so that in order to tilt and rotate against the wind pressure and to support such heavy reflector and extremely large and complicated mount is required. In addition a drive mechanism for driving such heavy mount consumes considerable power and must drive the mount and hence the reflector antenna with a very high degree of accuracy. This means that the construction and weight of such reflector are limited by the construction of the mount to be used. The advancement of the satellite launching techniques has now made it possible to place a synchronous or geostational communictation satellite in a 24-hr equatorial orbit for worldwide communication systems. In this case, the geostational communication satellite drifts relative to an earth station within an extremely limited range of i a few degrees so that a directive antenna is required only to change its direction within a few degrees in response to the drift of the geostational communication satellite in order to track it. Therefore it is obviously no longer economical to use the antennas of the type which must be tilted and rotated as the communication satellite moves because they are so heavy and costly.

In order to track the geostational communication satellites there have been widely used the so-called Cassegrain antennas or dual reflector antennas of the type each comprising a main reflector incorporating a horn reflector of a primary radiator or feeder and a subreflector which is located in opposed and spaced-apart relation with the main reflector. To track the geostational communication satellite only the subreflector is inclined relative to the axis of the main reflector which is held stationary with other major components of the antenna so that the direction of the radiated beams may be varied only in response to the drift angle of the geostational communication satellite. Thus the directive antennas used in the earth stations for tracking the communication satellites have been remarkably simplified in construction and reduced in cost. In theCassegrain antennas, the reflecting surfaces of the main reflectors are paraboloidal while those of the subreflectors are hyperboloidal, and the fixed focal point is located at one point along the axis of the main reflector. Therefore when only the subreflector is inclined while the main reflector is held stationary, the initially designed geometrical optics conditions are not satisfied so that the gain is considerably reduced. Therefore it is impossible to use a Cassegrain antenna in such a manner that only the subreflector is inclined or tilted to track a geostational communication satellite.

It is therefore one of the objects of the present invention to provide a dual reflector antenna which is best adapted to track a geostational or synchronous communication satellite and in which only a subreflector is inclined while a main reflector is held stationary without causing an appreciable change in initially designed geometrical optics conditions so that the radiated beam may be directed into a desired direction without causing an appreciable gain reduction.

Briefly stated, in accordance with the present invention the reflector curves of a main reflector and a subreflector of a dual reflector antenna are so constructed that the radiated beams are in phase in two directions slightly deviated from the axis of the main reflector. The angle of inclination with respect to the main reflector of the subreflector is continuously varied and the distance between the main reflector and the subreflector is simultaneously varied in tracking a geostational or synchronous communication satellite in order to obtain the highest gain.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic view of a dual reflector antenna in accordance with the present invention for explanation of the underlying principle thereof;

FIG. 2 is a graph for explanation of the graphical method for constructing the reflector curves of a main reflector and a subreflector thereof;

FIG. 3 is a schematic view of a practical dual reflector antenna in accordance with the present invention which has been designed and manufactured based upon the principle to be described in detail hereinafter by reference to FIGS. 1 and 2;

FIGS. 4 a and 4 b illustrate the directivity characteristics thereof;

FIG. 5 illustrates in a simplified form the characteristic curves shown in FIG. 4;

FIG. 6 is a graph illustrating the directivity characteristics of the antenna shown in FIG. 3 (when the subreflector is displaced from its normal position in accordance with the present invention);

FIGS. 7 and 8 are schematic views illustrating two other embodiments of the present invention;

FIG. 9 is a schematic view illustrating a drive device for the subreflector; and

FIG. 10 is a block diagram of a control unit for controlling the drive device shown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 is illustrated diagrammatically a dual reflector antenna in accordance with the present invention comprising, as in the case of the prior art antenna, a main reflector l, a subreflector 2 and a primary radiator 3. The reflector curves of the main reflector l and the subreflector 2 are so defined that when the subre- .flector 2 is inclined through an angle (b about a point 5 on the center line or axis 4 of the main reflector l, the radiated beams may be all in phase in a direction at an angle 0 The surfaces of the main reflector l and the subreflector 2 are symmetrical about the center lines or axes 4 and 6 respectively so that when the "subreflector 2 is inclined through the radiated beams are in phase in a direction of -0.,. In summary,

flector 2 is inclined at the radiated beams are directed in the direction 0 The gain of the radiated beams becomes maximum at :9 When the angle of inclination of the subreflector 2 is between tqb the radiated beams are out of phase, thus resulting in a lower gain. However, when as the angle da is small, the phase difference is also small so that the gain will not be appreciably reduced. The above described directivity is also true for reception.

Therefore when the reflector curves of the main reflector l and the subreflector 2 are so designed that the angles ":6 at which the gain of the radiated beam becomes maximum are made substantially equal to the maximum drift angle of the geostational satellite, it becomes possible to track the geostational satellite without causing the appreciable decrease in gain of the radiated beams when the subreflector 2 is continuously inclined. The drift angle of the geostational satellite is only a few degress so that the angle it) are also a few degrees at which no appreciable gain reduction will occur.

The reflector curves of the main reflector 1 and the subreflector 2 are not simple parabolas or hyperbolas and are obtained graphically as will be described in detail hereinafter. The method will be described when the dual reflector antenna uses a primary radiator which radiates plane waves, but it is understood that this method is also applied when a primary radiator radiating spherical waves is used.

Now referring to FIG. 2, the axis of the main reflector 10 is selected as the X-axis, and the subreflector 11 is inclined through an angle 45 about a point 12 on the X-axis. The point of intersection between the X-axis and the subreflector 11 which is in a normal position, that is when it is not inclined at all, is designated by 13 (14,, v That is, the center of the subreflector is located at the point 13 The wavefront 14 directed at an angle 0 which intersects the X-axis at 12.

First let us investigate the waves from the primary radiator reaching the point 13' (u,', v,') which is the center of the subreflector 11. when it is inclined at 4) The primary radiator radiates the plane waves as described above so that the waves propagate in parallel with the X-axis. The reflector curve of the subreflector 11 is .perpendicular to the X-axis when it is in normal position, that is when its center is at 13 so that the reflector curve is perpendicular to the line connecting the point 12 with the point 13 (14,, v when the subreflector 11 is inclined through an angle Therefore the waves reaching the point 13' are reflected at an angle 28, relative to the X-axis toward the main reflector 10. The waves are reflected again at the point (x y,) of the main reflector 10 to be made incident at right angles to the wavefront 14. Therefore from the law of reflection an angle of inclination a, of the main reflector 10 at the point (x,, y,) is obtained. Thus one point of the reflector curve of the main reflector 10 is obtained. The

length of the path of the waves from the point (0, v,) to the wave front 14 through the points (14 v and (x,, y,) is designated by l.

Since the reflector curve of the main reflector 10 is symmetrical about the X-axis, there is a point (x,, y,) on the reflector curve which is symmetrical about the X-axis with respect to the point (x y The coordinate relations are given by x, x, and y, y,. The angle 01 of inclination of the main reflector at this point (x y is equal to a,. Conversely, the ray from the wave front 14 reaching the point (x,, y,) is reflected at an angle of reflection which is determined by 0 and 0: The waves are further reflected by the subreflector 11 and are propagated in parallel with the X-axis reaching the Y-axis where x 0. In this case the length of the path of the waves from the wave front 14 must be equal to I so that the second point (14 v,') on the subreflector 11 is determined and an angle [3 of inclination of the subreflector 11 at this point is also obtained. From this point (142 v and the angle 3 of inclination, the corresponding point (14 v and the angle of inclination of the subreflector 11 when it is not inclined may be obtained.

The point (u v which is symmetrical about the X- axis with respect to the point (a v may be easily obtained and the angle B of inclination of the subreflector at this point may be obtained from the relationship.

In a similar manner the symmetrical point (u v and the inclination 8;, at this point when the subreflector is inclined through d 0 may be obtained. Then the angle of reflection at the point (u v;,) of the waves which are radiated from the primary radiator and propagated I in parallel with the X-axis can be obtained. The reflected waves are also reflected by the main reflector l0 and are made incident to the wave front 14 at right angles. Since the length of the path 1 remains unchanged, the point of reflection (x y on the main reflector and the angle a of inclination thereof at this point may be obtained.

In summary, the above described graphical techniques for obtaining the reflector curves of the main reflector and the subreflector may be given by the following relationship:

cos 0,, y sin 0 reflector I0 and the subreflector 11 are obtained. The reflector curves are obtained by connecting these points. The dual reflector antenna constructed by the method described above by reference to FIG. 2 may track only one plane, but it is easily understood that a surface of revolution may be generated by revolving each reflector curve thus obtained about the axis so that the duel reflector antenna having the reflector surfaces which are symmetrical about the axes may be provided.

FIG. 3 is a diagrammatic view of a dual reflector antenna in accorance with the present invention which is designed for practical purposes based upon the princi-.

ple described by reference to FIGS. 1 and 2. A main reflector 20 has a diameter of 1,200 mm and an aperture angle of about 120 while a subreflector 21 has a diameter of 190.3 mm. The reflector surfaces of the main reflector 20 and the subreflector 21 are obtained by generating the surfaces of revolution by revolving the reflector curves which are so constructed that when the subreflector 21 is inclined through 6(=4 in FIG. 3) the desired phase of the radiated beam is inclined through 2 6 in FIG. 3). In a primary radiator 22, there is used a conical horn reflector having an aperture diameter of 168 mm and an aperture angle of 20. The primary radiator 22 is located at the center of the main reflector 20 which is spaced apart from the center 23 of rotation of the subreflector 21 by 431.50 mm. When the subreflector 21 is in the position at which it is not inclined relative to the main reflector 20, its center is spaced apart from the center of the main reflector 20 by a distance of 411.14 mm.

FIG. 4 is a graph illustrating the directivity characteristics of the dual reflector antenna shown in FIG. 3. The angle 0 is plotted against the abscissa while the relative field intensity is plotted against the ordinate. The polarized waves indicated by the arrow were used in measurement in a plane perpendicular to the axis of the horn reflector 22. That is, FIG. 4-(a) shows the change in directivity when the angle 4) of the subreflector is changed in the positive direction while FIG. 4-(b) shows the change in directivity when the angle (b is changed in the negative direction. As the inventors expected the dual reflector antenna shown in FIG. 3 has the maximum gain when the subreflector is inclined slightly toward the positive or negative direction. It is seen that the gain is not appreciably reduced between the maximum gain directions. It is also proved that the steerable angle range by the subreflector is remarkably wider than that of the Cassegrain antenna.

The directivity of the dual reflector antenna shown in FIG. 3 may be illustrated in a simpler form as shown in FIG. 5. The angle 0 is plotted against the abscissa while the field intensity is plotted against the ordinate. When the reflector curves of the main reflector 20 and the subreflector 21 are so designed that the radiated beams are all in phase in the directions inclined at :0 when the subreflector 21 is inclined at iqi the directivity curve indicated by A is obtained when the subreflector 21 is inclined at while the directivity curve indicated by B is obtained when the subreflector is inclined at On the other hand, when the subreflector 21 is not inclined with respect to the main reflector 20, the directivity curve indicated by C is obtained. In this case, it is seen that the gain is slightly lowered, but when the angle 0 is not so great, no appreciable decreasein gain is observed. The broken curve D is the values of the gain when the angle 0 of inclination of the radiated beam (plotted against the abscissa) was continuously changed. The broken curve E indicates the directivity when the subreflector was displaced away from the normal position by 6 mm while the dotted curve F indicates the directivity when it is displaced by 9 mm. From FIG. 6 it is seen that the gain may be increased when the subreflector is displaced from its normal position because the surfaces of the main reflector and the subreflector of the antenna shown in FIG. 3 are the surfaces of revolution generated by revolving the plane reflector curves about the axes so that some aberrations occur in the direction perpendicular to the direction in which the subreflector is inclined. The principal aberration is the spherical aberration so that when the subreflector is moved away from the main reflector, the spherical aberration is decreased. Consequently the gain is improved. 1

Therefore from an inspection of FIG. 6 it is seen that in practice when the geostational satellite is tracked with the dual reflector antenna shown in FIG. 3, it is necessary not only to incline the subreflector but also to displace it along the axis forwardly or backwardly. Alternatively the position of the subreflector is so selected that the desired directivity curve E such as shown in FIG. 6 may be attained.

Another embodiment of the present invention schematically illustrated in FIG. 7 is adapted to track a specified geostational satellite. A main reflector 32 is supported by supports 31 fixed to a foundation 30 in such a manner that the reflecting surface of the main reflector 32 may be directed toward the specified satellite. A subreflector 35 is mounted in opposed relation with the main reflector 32 on a drive or steering device 34 which in turn is supported by supports 33 protruding from the main reflector 32. A primary radiator an'd'a waveguide feeder are designated by 36 and 37 respectively. It is of course apparent that the reflector curves of the main reflector 32 and the subreflector 35 are so constructed that the radiated beams are in phase in a direction slightly deviated from the axis of the main reflector 32. In tracking the satellite, the subreflector 35 is rotated by the drive or steering device 34 to be described in more detail hereinafter in response to the drift of the geostational satellite. Since the main reflector is fixed, only the specified geostational satellite may be tracked.

However, it is sometimes desired in practice to selectively track a plurality of geostational satellites launched in the equatorial orbital plane. A dual reflector antenna in accordance with the present invention shown in FIG. 8 is especially adapted for this purpose. In this antenna, a main reflector 40 is mounted on a support 47 for rotation, and a subreflector 43 is movably mounted on a drive or tracking device 43 which in turn is supported by supports 41 protruding from the main reflector 40. A pair of spaced apart semicircular gears 44 and 45 are fixed to the back of the main reflector 40 and are in mesh with a pair of gears 48 and 49 which are rotatably carried by the support 47. Therefore when a motor 50 mounted on the support 47 which in turn is erected upon a foundation 46 and coupled to the gear 49 rotates, the main reflector 40 is rotated about the axis 51 in the directions indicated by the double-pointed arrow. Since the axis 51 is directed to the north, the antenna may track a plurality of geostational satellites above an equatorial orbital plane, and the desired geostational satellite may be tracked by rotating the subreflector 43 by the drive device 42 as in the case of the antenna shown in FIG. 7. In the instant embodiment, a horn reflector 52 of a primary radiator is located at the center of the main reflector 40 in such a manner that the axis of thecone portion of the horn reflector is coaxial with the axis 51. Since the horn reflector 52 rotates in unison with the main reflector, it is coupled to a waveguide feeder 53 through a rotary joint 54.

The subreflectors of the antennas shown in FIGS. 7 and 8 are driven by the drive device of the type shown in FIG. 9. A subreflector 60 is rotated about the point 61 and is provided with an arcuate guide rail 62 whose center coincides with the point 61. The guide rail 62 is supported through a pair of guide members 63 and 64 by a support 65. The screw-threaded portion 66 of the guide rail 62 is in mesh with a gear 68 which is directly coupled to a motor 67 mounted upon the support .65 so that upon rotation of the motor 67 its rotation is transmitted to the guide rail 62 and hence to the subreflector 60 through the engagement of gear 68' with the screw-threaded portion 66. Thus the subreflector 60 is rotated. The supporting arm 65 in turn is supported by supporting arm 69 through a pair of guide members 72 and 73 which are securely fixed to the supporting arm 65 and into which are fitted guide rail portions 70 and 71 of the supporting arm 69. A rack 74 is extended from the midpoint of the supporting arm 65 and is in mesh with a gear 77 driven by a motor 76 which in turn is carried by the supporting arm 69. Therefore upon rotation of the motor 76 the supporting arm 65 is moved forwardly or backwardly in the directions indicated by the double-pointed arrow. Consequently the subreflector 60 which is carried by the supporting arm 65 is moved forwardly or backwardly along the axis of a main reflector (not shown in FIG. 9). The supporting arm 69 is fixed to the rotary shaft of a motor 79 which in turn is carried by a support 80 extending from the main reflector (not shown). The axis of rotation of the motor 79 is coaxial with the axis of the main reflector 78 so that upon rotation of the motor 79 the subreflector 60 rotates about the axis 78. In short, the subreflector 60 is inclined relative to the axis 78 by the motor 67, displaced forwardly or backwardly along the axis 78 by the motor 76 and rotated about the axis 78 by the motor 79.

The three motors for driving the subreflector in three directions described above are controlled by a control unit whose block diagram is shown in FIG. 10. A tracking direction error detector 90 detects the error between the drift angle of the geostational satellite and the angle of direction of the radiated beam. The error signals generated by the detector 90 consist of a signal representative of an error about the axis of the antenna and a signal representative of an error in the direction perpendicular to the axis. These two error signals are fed into error signal generators 91 and 92 respectively.

In response to a control signal from the error signal generator 91, a control circuit 93 controls the motor 79 to precisely control the angular position of the subreflector about the axis. The signal representative of an error in the direction perpendicular to the axis is fed into an analog computer 94 in which the signal is resolved into a component in response to which the subreflector is to be displaced along the axis forwardly or backwardly and a component in response which the subreflector is to be inclined relative to the axis. The former component is applied to a control circuit 95 to drive the motor 76 while the latter component is ap-- plied to a control circuit 96 to drive the motor 67. Thus any of the desired directivity characteristic curves shown in FIG. 6 may be attained.

It is seen that when the subreflector is previously fixed to such a position in which the directivity curve E shown in FIG. 6 is attained, the mechanism for displacing the subreflector forwardly or backwardly along the axis may be eliminated. Thus the drive device shown in FIG. 10 may be further simplified.

I claim:

1. A dual reflector antenna comprising a main reflector and a subreflector for reflecting therebetween signals to be radiated or received, and a primary radiator located along the central axis of said main reflector between said main reflector and said subreflector, said main reflector having a continuous curved surface symmetrical about said central axis, said subreflector having a continuous curved surface which in one position thereof is symmetrical about said central axis, said main and subreflectors having non-parabolic and nonhyperbolic curvatures whereby radiated waves are in phase in a direction plus or minus 0 deviating from said axis when the axis of the subreflector is at a small angle plus or minus lilo respectively with respect to said given axis.

' 2. The antenna of claim 1 further comprising means for changing the angle of inclination between said central axis and the axis of said subreflector.

3. The antenna of claim 2 further comprising means for simultaneously varying the distance between the main reflector and subreflector with changes in said angle of inclination.

4. The antenna of claim 3 comprising a first supporting structure for mounting said main reflector, a second supporting structure extending from said main reflector, and a drive device mounted in opposed relation with said main reflector upon said second supporting structure for mounting said subreflector, whereby the angle of inclination of said subreflector and the distance between said main reflector and said subreflector may be varied by said drive device.

5. The antenna of claim 1 wherein the curvatures of said main reflector and subreflector are graphically developed curvatures from the paths of plane waves originating at said primary radiator parallel to said central axis with the axis of the subreflector at an angle of th to the central axis, whereby the wave front of radiated waves after being reflected at the main reflector and sub-reflector is at an angle of 0., with respect to said central axis.

6. The antenna of claim I wherein the curvatures of said main reflector and subreflector are surfaces of revolution. 

1. A dual reflector antenna comprising a main reflector and a subreflector for reflecting therebetween signals to be radiated or received, and a primary radiator located along the central axis of said main reflector between said main reflector and said subreflector, said main reflector having a continuous curved surface symmetrical about said central axis, said subreflector having a continuous curved surface which in one position thereof is symmetrical about said central axis, said main and subreflectors having non-parabolic and non-hyperbolic curvatures whereby radiated waves are in phase in a direction plus or minus theta 0 deviating from said axis when the axis of the subreflector is at a small angle plus or minus psi 0 respectively with respect to said given axis.
 2. The antenna of claim 1 further comprising means for changing the angle of inclination between said central axis and the axis of said subreflector.
 3. The antenna of claim 2 further comprising means for simultaneously varying the distance between the main reflector and subreflector with changes in said angle of inclination.
 4. The antenna of claim 3 comprising a first supporting structure for mounting said main reflector, a second supporting structure extending from said main reflector, and a drive device mounted in opposed relation with said main reflector upon said second supporting structure for mounting said subreflector, whereby the angle of inclination of said subreflector and the distance between said main reflector and said subreflector may be varied by said drive device.
 5. The antenna of claim 1 wherein the curvatures of said main reflector and subreflector are graphically developed curvatures from the paths of plane waves originating at said primary radiator parallel to said central axis with the axis of the subreflector at an angle of psi 0 to the central axis, whereby the wave front of radiated waves after being reflected at the main reflector and sub-reflector is at an angle of theta 0 with respect to said central axis.
 6. The antenna of claim 1 wherein the curvatures of said main reflector and subreflector are surfaces of revolution. 